Detonation of explosive charges is generally effected by means of detonators which are provided in a detonating relationship with the explosive charges. Such explosive charges usually comprise so-called “main” or “secondary” explosives.
In the mining industry, in particular, as well as in a number of other industries which rely on the use of explosives, e.g. the demolition industry, accurate control of explosives detonation is of great importance, for reasons including safety and accuracy of blasting operation.
Generally speaking, one can distinguish between two types of detonators namely electronic detonators and pyrotechnic detonators.
Electronic detonators, generally, effect detonation of an explosive with which they are in a detonating relationship by generating a voltage spark or plasma in proximity to the explosive. Such voltage spark or plasma is generated by the breakdown of a resistive element or bridge which is provided between two conducting electrodes. The resistive bridge and the electrodes are generally referred to collectively as a “fuse head” which is accommodated within a detonator housing. The plasma generates a shock wave which is transmitted to the proximate explosive and initiates the explosive.
Such electronic detonators generally provide accurate control over detonation, particularly as regards timing and delay properties thereof. However, electronic detonators are expensive to manufacture and difficult to use, requiring a separate or external power source and complex electronic transmission wire connections to allow transmission of electricity to the detonator and permit remote triggering thereof. In the applicant's experience, such detonator connections are prone to failure and may even result in premature initiation of the detonator and thus of the explosive, due to false stimuli, e.g. being provided by radio-frequency (rf) interference on the mining/demolition site.
In contrast to electronic detonators operating by means of an electronic delay system, pyrotechnic detonators employ a series of explosive charges that are located within a detonator housing to provide a desired detonating signal to the main explosive charge at a required timing and delay. The series of explosive charges generally includes (i) an initiating and sealing charge, also known as a priming charge, (ii) a timing charge, (iii) a primary charge and, optionally, (iv) a base charge. The initiating charge serves to initiate the explosive sequence in response to a shock signal transmitted thereto and also functions as a sealing charge which provides a seal to prevent blow-back inside the detonator housing. The initiating charge also initiates the timing charge which provides a desired burning delay for detonation. A timing charge, in turn, initiates the primary charge which either directly provides a detonation initiating signal to the main explosive charge, or initiates the base charge that, in turn, will provide the desired detonation initiating signal to the main explosive charge.
As alluded to above, initiation of the initiating charge of a pyrotechnic detonator is generally effected by imparting a shock signal to the detonator, typically being provided by one or more shock tubes which are located in an initiating relationship with the detonator. The initiating charge then typically comprises a sensitive explosive, initiation of which can be effected by a shock wave of sufficient magnitude. Shock tube is well known and widely used in the initiation of detonators; it comprises a hollow plastic tube lined with a layer of initiating or core explosive, typically comprising a mixture of HMX and aluminium metal powder. Upon ignition of the initiating (core) explosive, a small explosion propagates along the tube in the form of an advancing temperature/pressure wave front, typically at a rate of approximately 7000 ft/s (about 2000 m/s). Upon reaching the detonator, the pressure/temperature wave triggers or ignites the initiating/sealing charge in the detonator, which results in the sequence of ignitions mentioned above and thus eventually causing detonation of the main explosive charge. Although shock tube is economically attractive and easy to use, existing pyrotechnic-based detonator systems do not at all permit the same extent of control of detonation timing and delay which is achieved by using electronic detonators, as the timing and delay features are provided by the detonator explosive charge loading, instead of by electric components.
The present invention therefore seeks, broadly, to provide an approach to operating explosive detonators which addresses and at least partly alleviates the disadvantages associated with both pyrotechnic and electronic initiation of explosive detonators.